1. Technical Field
The present invention relates to the field of plasma cutting machines, and more particularly, to an electrode head of the plasma cutting machine.
2. Description of Related Art
Plasma cutting machines are machines using plasma cutting for the working of metals. The heat with high temperature produced by the plasma arc torch can be used for partially melting (and vaporizing) the metal workpiece, and then the melted metal can be removed by the plasma. They are commonly used for cutting metals which are hard to be cut. Especially, the plasma cutting machines work well toward the nonferrous metal, for example stainless steel, aluminum, copper, titanium, nickel. When cutting metals with relatively thin thickness, especially the plain carbon steel plate, plasma cutting achieves a 5˜6 times higher speed than hydrogen oxygen cutting; In addition, the cutting surface is smooth, the heat deformation is small and the heat affected zone can hardly be observed. Therefore, plasma cutting is commonly used for cutting metals in the metal processing industry.
The electrode head has great effect upon the cutting characteristics, cutting quality, and cutting speed of the plasma arc. The electrode head is normally an assembly of sheath, bearing, and electrode. Commonly, the assembly of the electrode head is conducted by interference fit for engaging the different parts. However, the electrode heads have to be made with great precision. It's not easy to process, assemble or disassemble the electrode head by using interference fit. There are three basic methods for assembly with interference fit:                (1) Press fit method: Press fit is achieved with presses that can press the parts together with very large amounts of force. The presses may operate by hammers or press machines. However, extra jig is necessary for fixing the workpieces in this method. It's not easy to install the jig or to fix the workpieces on the jig when the workpieces have relatively small size. In addition, the compression occurred at once in the procedure of press-in loosens the engagement between the jig and the workpieces, and it conducts defects of the electrode heads. Therefore, using press fit method to assemble the electrode head with requirement of high precision and high accuracy is suffered from frequent defective products and the higher cost produced thereby.        (2) Thermal expansion method: Most materials expand when heated and shrink when cooled. In this method, the sheath is heated to expanse the hollow core of it, and the rod-shaped electrode is assembled into the hollow core of the sheath while hot, then they are allowed to cool and contract back to their former size. The common heating method is to put the sheath in to the hot water (80° C.˜100° C.) or hot oil (90° C.˜320° C.).        (3) Thermal contraction method: Alternatively, the rod-shaped electrode may be cooled before assembly such that it slides easily into the hollow core of the sheath. Upon warming, it expands and interferes. Cooling is often conducted by dry ice or liquid nitrogen. In the thermal expansion or contraction method, an extra thermal treatment is necessary, therefore the assembly of the electrode head becomes more complicated and difficult.        
Therefore, an improved electrode head assembly method is needed in this field of industry to resolve the problems in the prior arts for correctly engaging all the parts of the electrode with more simple process and less defective products.